Concept 22
DNA words are three letters long.

The genetic code had to be a "language" — using the DNA alphabet of A, T, C, and G — that produced enough DNA "words" to specify each of the 20 known amino acids. Simple math showed that only 16 words are possible from a two-letter combination, but a three-letter code produces 64 words. Operating on the principle that the simplest solution is often correct, researchers assumed a three-letter code called a codon.

Research teams at University of British Columbia and the National Institutes of Health laboriously synthesized different RNA molecules, each a long strand composed of a single repeated codon. Then, each type of synthetic RNA was added to a cell-free translation system containing ribosomes, transfer RNAs, and amino acids. As predicted, each type of synthetic RNA produced a polypeptide chain composed of repeated units of a single amino acid. Several codons are "stop" signals and many amino acids are specified by several different codons, accounting for all 64 three-letter combinations.